1. Field of the Invention
The present invention relates to an exposure apparatus suitable for use as a step-and-repeat reduction projection exposure apparatus for producing a device, and also to a method of controlling such an exposure apparatus. More specifically, the present invention relates to an exposure apparatus provided with an auto-focusing apparatus for making a focus adjustment by positioning the surface of each exposure area on a semiconductor wafer placed on a wafer stage with respect to the projection image plane of a reduction projection is lens system (projection optical system) so that a precisely focused image is formed on each exposure area, and also to a method of controlling such an exposure apparatus.
2. Description of the Related Art
In a known step-and-repeat projection exposure apparatus, the focus adjustment is generally accomplished by detecting a positional deviation of the exposure surface from the projection image plane and correcting the detected deviation each time an XY stage is moved to a location where an area to be exposed (exposure shot) is just below a projection lens.
When a small-sized pattern is transferred through a reduction projection optical system including a lens with a large diameter and thus a large numerical aperture, the depth of focus decreases with the increase in the diameter of the lens. The result of the reduction in the depth of focus is that the image transfer performance becomes sensitive to a focus error due to the non-uniformity of the surface to be exposed and/or due to the coating conditions of a photoresist. To avoid such a problem, a focus adjustment must be made for each exposure area. The focus adjustment mechanism generally includes an XY stage that is moved in a step and repeat fashion so that an area to be exposed comes to a location just below a projection lens; an auto-focus sensor for detecting a focus error between the image plane and the surface of the exposure area located just below the projection lens; a Z stage for correcting the focus error detected by the sensor; and an auto-focus control system for controlling the above-described elements.
The sequence of exposing each exposure area is performed as follows.
1. A wafer is moved (one step) by driving the XY stage so that an area to be exposed (exposure area) comes to a location just below the lens.
2. The focus error or the difference between the image plane of the projection optical system and the exposure area of the wafer is detected using the focus sensor.
3. The detected focus error is corrected by moving the Z stage in a direction along the optical axis.
4. Exposure is performed.
Each of a plurality of exposure areas is exposed in the process including steps 1 to 4 described above. That is, the above-described sequence including steps 1 to 4 is repeated as many times as there are exposure areas on the wafer placed on the stage.
In recent years, semiconductor devices have been produced using a wafer with an increasingly large size. Thus, the exposure apparatus is required to expose an increasingly greater number of exposure areas (shots) for each wafer. It is desired, thus, to improve the throughput of the step-and-repeat exposure apparatus. Furthermore, as the line width of a pattern to be transferred has become smaller, higher precision control is required in the focus adjustment in a shorter time.
However, in the step-and-repeat reduction exposure apparatus, it is required to move the XY stage steps by step so that a plurality of exposure areas (shots) on a wafer come one after another to a location just below the projection lens for exposure. A focus error between the projection image plane and the exposure shot occurs each time the XY stage is moved because of the difference between the traveling plane of the XY stage and the projection image plane.
The focus error between the projection image plane S and the exposure shot, including a focus error due to the nonuniformity of the surface of exposure areas, is detected by the auto-focus sensor and corrected by the Z stage. However, if the focus error is too large, the limitation in the adjustment capability of the Z stage results in a residual focus error greater than the upper limit allowed in the pattern-transfer process. One known technique to avoid the above problem is to repeatedly perform the detection and correction of the focus error using a feedback control technique until the focus error finally falls within the allowable range. However, the focus adjustment using the feedback control technique requires a long time, depending on the amount of the focus error. If the focus adjustment is performed using the auto-focus sensor and the Z stage without using the feedback control to improve the throughput, there can be a risk that the focus accuracy becomes lower than the required limit. For the reasons described above, the amount of correction in the focus-adjustment process should be as small as possible.
The focus error caused by the difference between the stage traveling plane and the projection image plane is proportional to the moving distance of the XY stage. To place a wafer with an increasing large size, it is required that the XY stage have an increasingly large size. As a result, the reduction in the focus error caused by the difference between the stage traveling plane and the projection image plane has become an important factor to achieve an exposure apparatus with sufficiently high performance.
In view of the above, it is an object of the present invention to provide a stage apparatus and an exposure apparatus capable of rapidly positioning the stage and making focus adjustment, and also a method of controlling such a stage and an exposure apparatus.
It is another object of the present invention to provide a technique to improve the throughput and the accuracy of the focus adjustment performed for each exposure shot using a step-and-repeat exposure apparatus.
It is still another object of the present invention to provide a technique to theoretically predict a focus error and correct the predicted focus error during the movement of the stage thereby achieving rapid positioning of the stage.
According to an aspect of the present invention, to achieve the above objects, there is provided a stage apparatus including: a stage movable in a first direction parallel to a first plane and also in a second direction substantially perpendicular to the first plane; first moving means for moving the stage by a specified distance in the first direction; calculation means for calculating a deviation in the second direction from a reference plane which will occur as a result of the movement of the stage accomplished by the first moving means, the calculation being performed based on the specified distance and the angle between the reference plane and the first plane; and second moving means for moving the stage in the second direction during the movement of the stage accomplished by the first moving means, so as to compensate for the deviation calculated by the calculation means.
The present invention also provides an exposure apparatus including the stage apparatus described above.
Furthermore, the invention provides a method of controlling the stage apparatus and the exposure apparatus.
Still furthermore, the invention provides a method of producing a device using the method of controlling the exposure apparatus.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.